A cellular network may include a push-to-talk (PTT) server for enabling multiple mobile stations to participate in a group conference call. In some examples, the group conference call may be generally referred to as a PTT over cellular (PoC) call. Typically, a PoC based system is half-duplex in nature, similar to 2-way radio communications where one mobile station user talks at a time and the other mobile station users listen. The PTT server enables a PoC call by handling conference setup signaling and, in some examples, media stream replication and distribution. In a packet data network, such as the Internet, the conference setup signaling may be carried out, for example, with session initiation protocol (SIP) signaling and the replicated media streams may be real time transport protocol (RTP) based.
When an originating mobile station attempts a PoC call with multiple target mobile stations, the originating mobile station may send an originating SIP INVITE message to the PTT server. The originating SIP INVITE, for example, contains identifying information associated with the target mobile stations, such as phone numbers, email addresses, or a group number. The originating SIP INVITE message also includes media stream parameters (i.e., media type and format, transport protocol, transport port number, etc.). The identifying information and the media stream parameters may be located within the body of the originating SIP INVITE message according to a protocol commonly referred to as the session description protocol (SDP).
When the PTT server receives the originating SIP INVITE message, the PTT server uses the information within the originating SIP INVITE message to create a SIP INVITE message destined to each of the identified target mobile stations. Each SIP INVITE message includes the destination address of a target mobile station and the media stream parameters that were located in the originating SIP INVITE message. The PTT server forwards these generated SIP INVITE messages to one or more radio access networks (RANs) that serves the target mobile stations. In particular, the PTT server may forward the SIP INVITE messages by way of packet data serving node (PDSN) associated with a given RAN and thus initiate a packet data session between the originating mobile station and the target mobile stations.
When a RAN serving a target mobile station receives the SIP INVITE message sent from the PTT server, the RAN will verify that a target mobile station is not dormant (i.e., the target mobile station is assigned a traffic channel). If the target mobile station has an assigned air interface traffic channel, the RAN will forward the SIP INVITE message over the traffic channel to the target mobile station. If, however, the target mobile station is dormant, the RAN will first page the target mobile station and assign a traffic channel to the target mobile station. After the traffic channel is assigned, the RAN will transmit the SIP INVITE message to the target mobile station.
Responsive to receiving a SIP INVITE message, the target mobile station generates a SIP OK message, acknowledging desired media stream parameters. The SIP OK message is sent over an assigned traffic channel back to the PTT server. The PTT server, in response, may send a SIP ACK message to each of the target mobile stations, and media streaming to the target mobile stations may begin.